Here are some reviews of improv activities from websites by one of my Drama Games students at HOL.edu.
In this site I found 'Scare off the Bench'. This game would appeal to my high schoolers as it involves repulsing the actor on the bench to the point of not being able to act anymore. This would be a challenge I could see many of them relishing!
This is a Canadian improv games website that had new games that I have never heard of before. 'Samurai' would be a hit with some of my high school kids who really love to play battle games. This game is played with pretend swords and dueling movements.
As a music teacher I was excited to find this site! There are multiple warm-ups, games, and activities here that I could use in my classes. Here are a few: 'Harmony Duets'. One player sings a line and then repeats it as another harmonizes to their melody. Challenging for the ear!
'Band' would have players miming playing their instrument in a band or orchestra as a conductor directs them and trys to direct them how to nuance their 'music' as in slower, more intense, lighter, faster, etc. Also in this site I found 'Hotspot' where the player in the center sings a familiar song until they can't go on, then another player steps in to morph it into their own song.
This site also had many music related games and activities. 'American Idol' game has a host character, a mean judge, a nice judge and a contestant who makes up an act. Then the judges improv their replies. It would be good practice in acting out personal interactions. 'Balladeer' has a player sing a story line while other actors out it out. This would work well to help singers work diction so they can be understood.
For more improv games go to http://www.freedrama.net/improv.html